The present invention relates to a room temperature curable polyorganosiloxane composition which exhibits a high strength, particularly a high tear strength, after curing.
Even if silicone polymers per se are crosslinked and cured, they merely afford very fragile rubbers, due to their characteristics (low intermolecular force and amorphous structure). Without the addition of a reinforcing filler, it is impossible to obtain satisfactory physical characteristics, and the products obtained are not employable actually. Therefore, various kinds of reinforcing fillers have heretofore been adopted for silicone polymers and the resulting products have actually been commercialized.
The reinforcing effect of a filler in a silicone rubber is very outstanding, not recognized in other synthetic rubbers, and it is several times as high as other synthetic rubbers. For this reason, various reinforcing fillers have heretofore been added to silicone products. Examples are fine silica powder, diatomaceous earth, quartz powder, calcium carbonate, zinc carbonate, magnesium carbonate and carbon black.
Above all, fine silica powder exhibits a particularly superior reinforcing effect, which effect is classified into a crystallization effect attained by the adsorption of silicone rubber to the silica surface and an effect attained by chemical bonding of silanols with each other on the silica surface. Reinforcement is effected by physical and chemical bonding with the silicone polymer to which the silica powder is added. Increasing the amount of silica used for enhancing its reinforcing effect also leads to a marked increase of viscosity and hence leads to deterioration in the handleability of the resulting silicone product itself. Thus, the amount of silica to be added has been restricted. For this reason, the silicone rubber with such fine silica powder incorporated therein has heretofore not been fully satisfactory in point of its mechanical characteristics. Moreover, the tear strength of the silicone rubber with such fine silica powder incorporated therein has heretofore not been improved, which is another drawback of the silicone rubber, and the improvement thereof has been desired particularly for a room temperature curing silicone rubber low in crosslinking density.
Due to the above-mentioned drawbacks, the silicone rubber, as a room temperature curing type product, has so far not been used for structural bonding and sealing.
Further, as a filler, pigment or additive, it has also been known to add titanium oxide to a silicone product, but by the addition of an ordinary form of titanium oxide, it has been impossible to obtain a satisfactory effect.
It is the object of the present invention to solve the above-mentioned problems of the prior art and provide a room temperature curable polyorganosiloxane capable of affording a cured product having superior mechanical characteristics, especially high tear strength, and also superior in processability.